Lenses have for centuries been used in imaging systems, focusing optics etc. Usually they are made from a material that is transparent in the wavelength range they are to be used, and they have polished surfaces according to some prescribed shape. Most lenses are formed in the shape of a disc with polished surfaces above and below. Typically, one or both of the polished surfaces take the form of part of a sphere with a radius of curvature equal to or larger than the radius of the disc. If only one of the surfaces is spherical, the opposite surface is usually flat. Common materials used to manufacture lenses are glasses, crystals transparent in the visible such as sapphire, and infrared transparent materials such as silicon, zinc selenide, zinc sulphide and germanium.
There are certain applications which require particular material properties of the lens. Any lens which is used in the manipulation of high intensity laser light needs to have a high optical damage threshold. A high thermal conductivity is also beneficial as this minimises the temperature variation within the lens, thereby reducing distortion. In certain applications it is important that the material used simultaneously displays a high transmission and a high refractive index at the wavelength of light to be focussed by the lens.
Diamond displays material properties useful in lenses for such applications, and accordingly the ability to manufacture a diamond lens to a high specification is desirable. Lenses formed from diamond have been made in the past but are rare due to the difficulties involved in polishing such lenses to provide the necessary optical properties.
A type of lens which would particularly benefit from being formed of diamond is a Solid Immersion Lens (SIL) of the type used in optical pickup devices for reading information from digital media such as DVDs. US-A-2004/0047270 describes an information recording and/or reproducing apparatus which aims to improve recording and reproduction capacity on an optical disc. The apparatus includes a hemispherical or super-hemispherical SIL as part of a converging lens unit (a “super-hemispherical” surface refers to the shape of the larger portion of a sphere divided by a plane not including the centre of the sphere). The SIL must be formed of a highly refractive and transmissive material, and diamond is suggested as a potential material from which such a SIL might be formed. However, no information is provided as to how a diamond SIL might be produced.
As described in Optical Near-Field Recording, by J. Tominaga and T. Nakano, Springer-Verlag, Berlin Heidelberg, 2005, ISBN 3-540-22128-X, a SIL should have a shape which is hemispherical or super-hemispherical with a polished flat side opposite the spherical surface. Its thickness, t, is determined to be either equal to the radius, r, of the SIL in the case of the hemispherical SILt=r  (1)or for a super-hemispherical SIL
                    t        =                  r          ⁡                      (                          1              +                              1                /                n                                      )                                              (        2        )            where n is the index of refraction of the material from which the SIL is made at the wavelength of radiation used. Other designs using SILs with thickness slightly modified from the values given in Equations 1 and 2 have also been considered. In such cases, a DVD may include a layer carrying information bits buried under a relatively thick (several micrometer) top layer. In order to get diffraction-limited performance of the overall optical system, the thickness of the SIL must be reduced, and the focusing and spherical aberration of the optical system preceding the SIL adjusted, to compensate for the defocus and the spherical aberration of this top layer.
A SIL used for such an application needs to conform to very high demands on the accuracy of the spherical surface in terms of deviations from an ideal spherical surface and the roughness of the polished surface. In addition the thickness of the SIL needs to be controlled to fractions of a micrometer. The reason for these stringent requirements is the need for an optical system with a so-called diffraction limited spot size in order to obtain the highest possible storage density. To date, it has not been possible to polish a diamond lens having a sufficiently small radius of curvature, or with the required optical properties, for use as a SIL.
Lens polishing is traditionally achieved using one of two distinct principles:                1) Single point turning: In this fairly recent method lenses can be formed of soft materials such as plastics, or infrared transparent materials such as germanium or zinc selenide. Material is removed with a very sharp, very hard turning tool, almost always a diamond tip. By prescribing the motion of the tool tip relative to the workpiece, a surface can be produced on the workpiece with a prescribed shape. This surface need not be spherical but can have an aspheric shape, such as a paraboloid or ellipsoid shape. The accuracy of the shape is mainly determined by the accuracy and stability of the stages on which the turning tool is mounted. Air-bearings are usually used to insulate the work piece and turning tool from vibrations in the environment. This system is not suitable for use in shaping diamond surfaces, because the inevitable wear of the turning tool is much larger than the removal rate of material from the workpiece due to the large ratio of the surface area of the workpiece relative to the area of the tool tip.        2) The more traditional polishing technique for glass optics relies on a polishing powder filled pitch or felt cup or wheel which rotates around its axis. The glass workpiece is pressed onto the cup and is polished by a random motion relative to the cup. Traditional materials used as powders in such polishing include corundum, silicon carbide and diamond. Due to the random motion of the cup or wheel relative to the workpiece, the surface of the workpiece will eventually gain a spherical shape, and very accurate lenses can be produced in this manner. Diamond is traditionally polished using a modified version of this method, but there are limits to the lenses which can be produced in this way, as discussed in more detail below.        
It is well known that diamond is the hardest material available in nature. Diamond is therefore traditionally polished using rapidly rotating cast-iron or bronze wheels or “scaifes” impregnated with a fine diamond powder. Scaife technology enables the production of finely polished diamond surfaces. Unlike glass, which is an isotropic amorphous material (having the short range order of a liquid but no long range order), diamond is a cubic crystal. This means that in certain planes relative to the crystal axes it is relatively easy to polish diamond, while in other directions polishing is almost impossible by traditional methods. In particular, the “soft” (easily polished) planes coincide with (for example) the so-called 2-point and 4-point planes, known in crystallographic terminology as {110} and {100} planes, while the so-called 3-point planes (the {111} planes) coincide with the “hard” or very difficult to polish planes.
More recent methods involve the use of polishing wheels comprising resin polymer filled with diamond powder and mounted on metal carrier wheels or cups, and this enables polishing of diamond in arbitrary directions. However, the effects of varying hardness in different directions are still clearly present, and the polishing speed therefore still varies with the orientation of the crystal. For most traditional diamond polishing only flat planes are polished, in which case the speed of polishing is uniform over the area of the plane, resulting in uniform removal of material.
When polishing a spherical surface, as required by a lens, the polishing wheel or cup must remove material from the lens over a range of orientations of the polishing wheel with respect to the crystal axes. Thus, even when using a resin-bond polishing wheel or cup, the removal rate will be non-uniform across the surface. This results in a surface having large deviations from the intended ideal spherical shape, especially if a substantial fraction of a hemisphere is required.
Diamond lenses have been produced both from single-crystalline and from poly-crystalline material. Where poly-crystalline material fabricated by Chemical Vapour Deposition (CVD) is used, it is possible to grow the diamond onto a pre-shaped surface. The as-grown diamond discs are then polished flat using standard techniques of resin-bond wheel polishing, or can be given a curved finish with a curved polishing wheel or cup. However this technique cannot be used for natural single-crystalline diamond, or for synthetic crystals grown using the High Pressure and High Temperature (HPHT) technique. Furthermore, it is also not appropriate for CVD-grown single-crystalline material. This is because single crystal CVD diamonds are grown on a substrate which is itself formed from diamond, for which pre-shaping would be just as difficult as for the CVD-grown material to be shaped. In addition, removing the CVD-grown diamond from the diamond substrate can only be done by sawing or laser cutting, and in any case diamond growth on a curved substrate results in a faceted final stone.
The only single-crystal diamond lenses produced until now have been made by polishing a curved surface onto the diamond using the techniques described above. The spherical surfaces of such lenses have radii of curvature much larger than half the diameter of the lens and are thus not suitable for use in SILs (where the radius of curvature is equal to half the diameter of the lens). Previous single crystal diamond lenses also have large deviations from an ideal spherical surface, depending on the fraction of a sphere that is polished.
In addition, when polishing flat diamond surfaces using traditional techniques, the diamond object is typically gripped in a device known as a “tang” and then manually pressed with considerable force against the surface of the rapidly rotating polishing scaife. However, no accurate control is available that enables removal of a layer of diamond having a prescribed thickness. It is therefore not easily possible to control the thickness of the lens to the tolerance required when fabricating a SIL.
When a SIL is used for DVD applications, diffraction-limited performance is required for the optical system including the diamond SIL at short UV range wavelengths, e.g. 405 nm and 266 nm. The only natural diamond material which allows transmission of such short wavelengths is diamond of type IIa, which has a very low concentration of nitrogen impurities, typically below 25 ppm. However, type IIa natural diamond frequently suffers from large stresses in the material due to a high density of extended crystal imperfections such as dislocations and stacking faults. Thus, due to stress induced variation in refractive index and stress induced birefringence in the bulk of the diamond material itself, type IIa natural diamond is generally not well suited to make such high quality SILs with diffraction-limited performance. At best, a very low material yield would result from selecting suitable parts out of type Ia diamonds. Due to these variations in crystal quality and differences in material properties due to natural variations, such SILs would have to be tested individually and would have individually different dimensions and shapes to compensate for the material's variability, making them unsuitable for mass production and usage.
In metrology, spherical tips are used to map out the shape and roughness of surfaces made from metals, glasses, ceramics, crystalline, and other materials, whose surface shape needs to be measured with high accuracy. The tip is usually mounted on a partially flexible arm, which is fitted with a highly sensitive detector for sensing any flexing of the arm. Typically, movements of the order of a few nanometers can be detected. At the start of a measurement the tip is brought in contact with the object to be measured. Usually either the arm with the tip or the object to be measured itself is mounted on a XYZ translation stage and/or a rotary stage and the tip or the object is displaced or rotated according to a pre-programmed motion. Deviations of the shape of the object from the shape described by the pre-programmed motion are then detected by the flexing of the arm on which the tip is mounted.
In typical metrology applications, since the object to be measured is in general non-spherical, different parts of the tip are in contact with the measurement object. In order to have a flexing of the measurement arm which is independent of the position on the tip, which is in contact with the object, the tip itself needs to be spherical to an accuracy which exceeds the accuracy of the motion so that the accuracy of the measurement is not adversely affected by the shape of the tip. Currently available tips suffer from the problem of wear, since the tip is in continuous sliding contact with the surface of the object. Especially when hard and/or rough materials are measured, the wear of the tips leads to rapid deterioration of the spherical shape of the tip. This leads to measurement errors when using these tips. In such cases, the tip must be replaced by a new and undamaged tip. This leads to a high cost of measurement caused by the high cost of the tips and the need to recalibrate each new tip. Another common problem with softer materials such as aluminium is a build-up, even during a single measurement, of the material from which the measurement object is made on the surface of the tip, thus leading to measurement errors.
Furthermore in metrology one may want to measure the size of a hole in a material by passing balls of different diameters through the hole and determining the maximum size ball that does pass through the hole. In that case it is important that the balls are exactly spherical, do not deform and have low wear characteristics. For this application a completely spherical ball is not necessarily required, instead a super-hemispherical surface may suffice.
Metrology tool tips and balls are currently made from hard materials such as tungsten carbide, ruby and sapphire but even these materials show wear. Diamond shows exceedingly low wear characteristics and would be an ideal material from which to manufacture metrology tips and/or measurement balls. Further advantages to the use of diamond are provided by its hardness and very low friction coefficient when in contact with most hard and soft materials, including diamond itself. This would make diamond the preferred material for use in a metrology tip or measurement ball. However the problem of shaping a diamond tip into a sphere or super-hemisphere to the required accuracy has until now precluded its use in this application.
Those skilled in the art will appreciate that there are other applications of reference spheres and super-hemispheres, for example in the calibration of callipers, and, generally in sets of three, for defining a plane on which a flat plate may be placed for measurement, for example in an interferometry setup.